Sign up to receive free email alerts when patent applications with chosen keywords are publishedSIGN UP

Abstract:

An optical system used with a laser apparatus may include a focusing
optical system, a beam splitter, and an optical sensor. The focusing
optical system has one or more focus, for focusing a laser beam outputted
from the laser apparatus. The beam splitter is disposed between the
focusing optical system and the one or more focus of the focusing optical
system. The optical sensor is disposed on a beam path of a laser beam
split by the beam splitter.

Claims:

1. An optical system used with a laser apparatus, the optical system
comprising: a focusing optical system having at least one focus, for
focusing a laser beam outputted from the laser apparatus; a beam splitter
disposed between the focusing optical system and the at least one focus
of the focusing optical system; and an optical sensor disposed on a beam
path of a laser beam split by the beam splitter.

2. The optical system according to claim 1, further comprising: a focus
position adjusting unit, disposed on a beam path of the laser beam
upstream from the focusing optical system, for adjusting at least one of
a beam axis and divergence of the laser beam; and a focus control unit
for controlling the focus position adjusting unit based on a detection
result by the optical sensor.

3. The optical system according to claim 2, wherein the optical sensor
detects a focus condition of the laser beam, and the focus control unit
controls the focus position adjusting unit based on the focus condition
detected by the optical sensor.

4. The optical system according to claim 3, wherein the focus condition
includes at least one of a focus position of the laser beam, divergence
of the laser beam, a position of a beam axis of the laser beam, and a
direction of the beam axis of the laser beam.

5. The optical system according to claim 1, further comprising a beam
path adjusting unit for making a beam path of a first laser beam
outputted from a first laser apparatus and a beam path of a second laser
beam outputted from a second laser apparatus coincide with each other.

6. The optical system according to claim 5, wherein the laser beam
includes the first and second laser beams, the beam splitter transmits
the first laser beam and reflects the second laser beam, and the optical
sensor is disposed on the beam path of the second laser beam reflected by
the beam splitter.

7. The optical system according to claim 1, wherein the focusing optical
system is a reflective type optical system.

8. The optical system according to claim 1, wherein the beam splitter
includes a diamond substrate.

9. An optical system used with first and second laser apparatuses, the
optical system comprising: a beam path adjusting unit for making a beam
path of a first laser beam outputted from the first laser apparatus and a
beam path of a second laser beam outputted from the second laser
apparatus coincide with each other; a focusing optical system having at
least one focus, for focusing the first laser beam outputted from the
first laser apparatus and the second laser beam outputted from the second
laser apparatus; an optical sensor for detecting a focus position of the
second laser beam; a focus position adjusting unit disposed on a beam
path of the first and second laser beams upstream from the focusing
optical system, for adjusting at least one of a beam axis and divergence
of the first and second laser beams; and a focus control unit for
controlling the focus position adjusting unit based on a detection result
by the optical sensor.

10. An extreme ultraviolet light generation system used with a laser
apparatus, the extreme ultraviolet light generation system comprising: a
chamber provided with at least one inlet through which a laser beam
outputted from the laser apparatus is introduced into the chamber; a
target supply unit for supplying a target material to a predetermined
region inside the chamber; a focusing optical system for focusing at
least part of the laser beam in the predetermined region; a beam splitter
disposed between the focusing optical system and the predetermined
region; an optical sensor disposed on beam path of a laser beam split by
the beam splitter; and a collector mirror for collecting extreme
ultraviolet light emitted as the target material is irradiated by the
laser beam inside the chamber.

11. The extreme ultraviolet light generation system according to claim
10, further comprising a beam path adjusting unit for making a beam path
of a first laser beam outputted from a first laser apparatus and a beam
path of a second laser beam outputted from a second laser apparatus
coincide with each other.

12. The extreme ultraviolet light generation system according to claim
11, wherein the laser beam includes the first and second laser beams, the
beam splitter transmits the first laser beam and reflects the second
laser beam, and the optical sensor is disposed on the beam path of the
second laser beam reflected by the beam splitter.

13. An extreme ultraviolet light generation system used with first and
second laser apparatuses, the extreme ultraviolet light generation system
comprising: a beam path adjusting unit for making a beam path of a first
laser beam outputted from the first laser apparatus and a beam path of a
second laser beam outputted from the second laser apparatus coincide with
each other; a chamber provided with at least one inlet through which the
first laser beam outputted from the first laser apparatus and the second
laser beam outputted from the second laser apparatus are introduced into
the chamber; a target supply unit for supplying a target material to a
predetermined region inside the chamber; a focusing optical system for
focusing the first and second laser beams in the predetermined region; an
optical sensor for detecting a focus position of the second laser beam; a
focus position adjusting unit, disposed on the beam path of the first and
second laser beams upstream from the focusing optical system, for
adjusting at least one of a beam axis and divergence of the first and
second laser beams; a focus control unit for controlling the focus
position adjusting unit based on a detection result by the optical
sensor; and a collector mirror for collecting extreme ultraviolet light
emitted as the target material is irradiated by the laser beam inside the
chamber.

[0003] This disclosure relates to an optical system and an extreme
ultraviolet (EUV) light generation system including the optical system.

[0004] 2. Related Art

[0005] In recent years, semiconductor production processes have become
capable of producing semiconductor devices with increasingly fine feature
sizes, as photolithography has been making rapid progress toward finer
fabrication. In the next generation of semiconductor production
processes, microfabrication with feature sizes of 60 nm to 45 nm, and
microfabrication with feature sizes of 32 nm or less, will be required.
In order to meet the demand for microfabrication with feature sizes of 32
nm or less, for example, an exposure apparatus is needed in which a
system for generating EUV light at a wavelength of approximately 13 nm is
combined with a reduced projection reflective optical system.

[0006] Three kinds of systems for generating EUV light have been known in
general, which include a LPP (Laser Produced Plasma) type system in which
plasma is generated by irradiating a target material with a laser beam, a
DPP (Discharge Produced Plasma) type system in which plasma is generated
by electric discharge, and a SR (Synchrotron Radiation) type system in
which orbital radiation is used.

SUMMARY

[0007] An optical system according to one aspect of this disclosure may be
used with a laser apparatus and may include: a focusing optical system
having at least one focus, for focusing a laser beam outputted from the
laser apparatus; a beam splitter disposed between the focusing optical
system and the at least one focus of the focusing optical system; and an
optical sensor disposed on a beam path of a laser beam split by the beam
splitter.

[0008] An optical system according to another aspect of this disclosure
may be used with first and second laser apparatuses and may include: a
beam path adjusting unit for making a beam path of a first laser beam
outputted from the first laser apparatus and a beam path of a second
laser beam outputted from the second laser apparatus coincide with each
other; a focusing optical system having at least one focus, for focusing
the first laser beam outputted from the first laser apparatus and the
second laser beam outputted from the second laser apparatus; an optical
sensor for detecting a focus position of the second laser beam; a focus
position adjusting unit disposed on a beam path of the first and second
laser beams upstream from the focusing optical system, for adjusting at
least one of a beam axis and divergence of the first and second laser
beams; and a focus control unit for controlling the focus position
adjusting unit based on a detection result by the optical sensor.

[0009] An extreme ultraviolet light generation system according to yet
another aspect of this disclosure may be used with a laser apparatus and
may include: a chamber provided with at least one inlet through which a
laser beam outputted from the laser apparatus is introduced into the
chamber; a target supply unit for supplying a target material to a
predetermined region inside the chamber; a focusing optical system for
focusing at least part of the laser beam in the predetermined region; a
beam splitter disposed between the focusing optical system and the
predetermined region; an optical sensor disposed on beam path of a laser
beam split by the beam splitter; and a collector mirror for collecting
extreme ultraviolet light emitted as the target material is irradiated by
the laser beam inside the chamber.

[0010] An extreme ultraviolet light generation system according to still
another aspect of this disclosure may be used with first and second laser
apparatuses and may include: a beam path adjusting unit for making a beam
path of a first laser beam outputted from the first laser apparatus and a
beam path of a second laser beam outputted from the second laser
apparatus coincide with each other; a chamber provided with at least one
inlet through which the first laser beam outputted from the first laser
apparatus and the second laser beam outputted from the second laser
apparatus are introduced into the chamber; a target supply unit for
supplying a target material to a predetermined region inside the chamber;
a focusing optical system for focusing the first and second laser beams
in the predetermined region; an optical sensor for detecting a focus
position of the second laser beam; a focus position adjusting unit,
disposed on the beam path of the first and second laser beams upstream
from the focusing optical system, for adjusting at least one of a beam
axis and divergence of the first and second laser beams; a focus control
unit for controlling the focus position adjusting unit based on a
detection result by the optical sensor; and a collector mirror for
collecting extreme ultraviolet light emitted as the target material is
irradiated by the laser beam inside the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Hereinafter, selected embodiments of this disclosure will be
described with reference to the accompanying drawings.

[0025]FIG. 14 schematically illustrates the configuration of an EUV light
generation system according to a sixth embodiment of this disclosure.

[0026]FIG. 15 is a perspective view illustrating the detailed
configuration of a mount with a tilt mechanism.

[0027]FIG. 16 illustrates a detailed configuration of a focus position
adjusting unit.

[0028] FIG. 17 illustrates a focus position adjusting unit according to a
modification.

[0029] FIG. 18 illustrates the focus position adjusting unit according to
the modification.

[0030]FIG. 19 illustrates the focus position adjusting unit according to
the modification.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] Hereinafter, selected embodiments of this disclosure will be
described in detail with reference to the accompanying drawings. The
embodiments to be described below are merely illustrative in nature and
do not limit the scope of this disclosure. Further, the configuration(s)
and operation(s) described in each embodiment are not all essential in
implementing this disclosure. Note that like elements are referenced by
like reference numerals and characters, and that duplicate descriptions
thereof will be omitted herein.

Contents

1. Summary

2. Terms

3. Overview of EUV Light Generation System

[0032] 3.1 Configuration

[0033] 3.2 Operation

[0034] 3.3 Burst Operation

4. EUV Light Generation System of First Embodiment

[0035] 4.1 Configuration

[0036] 4.2 Operation

[0037] 4.3 Effect

[0038] 4.4 Modification

5. EUV Light Generation System of Second Embodiment

[0039] 5.1 Configuration

[0040] 5.2 Operation

[0041] 5.3 Effect

6. EUV Light Generation System of Third Embodiment

[0042] 6.1 Configuration

[0043] 6.2 Operation

[0044] 6.3 Effect

7. EUV Light Generation System of Fourth Embodiment

[0045] 7.1 Configuration

[0046] 7.2 Operation

[0047] 7.3 Effect

8. Beam Splitters

[0048] 8.1 Diamond Beam Splitter

[0049] 8.2 Mirror Having Through-Hole

[0050] 8.3 Diffraction Grating

9. EUV Light Generation System of Fifth Embodiment

[0051] 9.1 Configuration

[0052] 9.2 Operation

[0053] 9.3 Effect

10. EUV Light Generation System of Sixth Embodiment

[0054] 10.1 Configuration

[0055] 10.2 Operation

[0056] 10.3 Effect

11. Supplementary Description

[0057] 11.1 Mount with Tilt Mechanism

[0058] 11.2 Focus Position Adjusting Unit

[0059] 11.3 Modification of Focus Position Adjusting Unit

1. Summary

[0060] An overview of the embodiments will be given below. When a laser
apparatus is operating in burst operation mode, a heat load on an optical
system for focusing a pulsed laser beam may fluctuate. In such a case,
the position or location at which the pulsed laser beam is focused by the
optical system may also fluctuate. When the position or location at which
the pulsed laser beam is focused fluctuates, the conversion efficiency
into the EUV light may decline. As a result, the EUV light may not be
supplied to the exposure apparatus stably or with constant power or
intensity.

[0061] Further, since the pulsed laser beam is not outputted during a rest
period of the burst operation mode, the position at which the pulsed
laser beam is focused cannot be obtained. The shift in the focus position
may be caused by vibration added to the focusing optical system. However,
since this change in the focus position cannot be detected during the
rest period of the burst operation mode, if the focus position changes
during the rest period, the EUV light will be generated under the changed
focus position condition after the burst operation mode is re-started.
Accordingly, the conversion efficiency into the EUV light may decline,
and in turn the EUV light may not be supplied to the exposure apparatus
stably or with constant power or intensity.

2. Terms

[0062] Terms used in this application may be interpreted as follows. The
term "droplet" may refer to one or more liquid droplet(s) of a molten
target material. Accordingly, the shape of a droplet may be substantially
spherical due to its surface tension. The term "plasma generation region"
may refer to a three-dimensional space in which plasma is to be
generated. The term "burst operation" may refer to an operation mode or
state in which a pulsed laser beam or pulsed EUV light is outputted at a
predetermined repetition rate during a predetermined period and the
pulsed laser beam or the pulsed EUV light is not outputted outside of the
predetermined period. The "predetermined repetition rate" does not have
to be a constant repetition rate but may, in some examples, be a
substantially constant repetition rate. In a beam path of a laser beam, a
direction or side closer to the laser apparatus is referred to as
"upstream," and a direction or side closer to the plasma generation
region is referred to as "downstream." The "focus condition" may include
the position or location at which the laser beam is focused, the
divergence of the laser beam, and the position and direction of the beam
axis of the laser beam.

3. Overview of EUV Light Generation System

3.1 Configuration

[0063] FIG. 1 schematically illustrates the configuration of an exemplary
LPP type EUV light generation system. An EUV light generation apparatus 1
may be used with at least one laser apparatus 3. In this application, a
system including the EUV light generation apparatus 1 and the laser
apparatus 3 may be referred to as an EUV light generation system 11. As
illustrated in FIG. 1 and described in detail below, the EUV light
generation apparatus 1 may include a chamber 2, a target supply unit
(droplet generator 26, for example), and so forth. The chamber 2 may be
airtightly sealed. The target supply unit may be mounted to the chamber 2
so as to pass through the wall of the chamber 2, for example. A target
material to be supplied by the target supply unit may include, but is not
limited to, tin, terbium, gadolinium, lithium, xenon, or any combination,
alloy, or mixture thereof.

[0064] The chamber 2 may have at least one through-hole formed in the wall
thereof. The through-hole may be covered with a window 21, and a pulsed
laser beam 32 may travel through the window 21 into the chamber 2. An EUV
collector mirror 23 having a spheroidal reflective surface may be
disposed inside the chamber 2, for example. The EUV collector mirror 23
may have first and second foci. The EUV collector mirror 23 may have a
multi-layered reflective film formed on a surface thereof, and the
reflective film can include molybdenum and silicon that is laminated in
alternate layers, for example. The EUV collector mirror 23 may preferably
be disposed such that the first focus thereof lies in a plasma generation
region 25 and the second focus thereof lies in an intermediate focus (IF)
region 292 defined by the specification of an exposure apparatus. The EUV
collector mirror 23 may have a through-hole 24 formed at the center
thereof, and a pulsed laser beam 33 may travel through the through-hole
24.

[0065] Referring again to FIG. 1, the EUV light generation system 11 may
include an EUV light generation control unit 5. Further, the EUV light
generation apparatus 1 may include a target sensor 4. The target sensor 4
may be equipped with an imaging function and may detect at least one of
the presence, trajectory, and position of a target.

[0066] Further, the EUV light generation apparatus 1 may include a
connection part 29 for allowing the interior of the chamber 2 and the
interior of the exposure apparatus 6 to be in communication with each
other. A wall 291 having an aperture may be disposed inside the
connection part 29. The wall 291 may be disposed such that the second
focus of the EUV collector mirror 23 lies in the aperture formed in the
wall 291.

[0067] Further, the EUV light generation system 1 may include a laser beam
direction control unit 34, a laser beam focusing mirror 22, and a target
collection unit 28 for collecting a target 27. The laser beam direction
control unit 34 may include an optical element for defining the direction
in which the laser beam travels and an actuator for adjusting the
position and the orientation (or posture) of the optical element.

3.2 Operation

[0068] With reference to FIG. 1, a pulsed laser beam 31 outputted from the
laser apparatus 3 may pass through the laser beam direction control unit
34, and may be outputted from the laser beam direction control unit 34 as
a pulsed laser beam 32 after having its direction optionally adjusted.
The pulsed laser beam 32 may travel through the window 21 and enter the
chamber 2. The pulsed laser beam 32 may travel inside the chamber 2 along
at least one beam path from the laser apparatus 3, be reflected by the
laser beam focusing mirror 22, and strike at least one target 27, as a
pulsed laser beam 33.

[0069] The droplet generator 26 may output the targets 27 toward the
plasma generation region 25 inside the chamber 2. The target 27 may be
irradiated by at least one pulse of the pulsed laser beam 33. The target
27, which has been irradiated by the pulsed laser beam 33, may be turned
into plasma, and rays of light including EUV light 251 may be emitted
from the plasma. The EUV light 251 may be reflected selectively by the
EUV collector mirror 23. EUV light 252 reflected by the EUV collector
mirror 23 may travel through the intermediate focus region 292 and be
outputted to the exposure apparatus 6. The target 27 may be irradiated by
multiple pulses included in the pulsed laser beam 33.

[0070] The EUV light generation control unit 5 may integrally control the
EUV light generation system 11. The EUV light generation control unit 5
may process image data of the droplet 27 captured by the target sensor 4.
Further, the EUV light generation control unit 5 may control at least one
of the timing at which the target 27 is outputted and the direction into
which the target 27 is outputted (e.g., the timing with which and/or
direction in which the target is outputted from droplet generator 26),
for example. Furthermore, the EUV light generation control unit 5 may
control at least one of the timing with which the laser apparatus 3
oscillates (e.g., by controlling laser apparatus 3), the direction in
which the pulsed laser beam 31 travels (e.g., by controlling laser beam
direction control unit 34), and the position at which the pulsed laser
beam 33 is focused (e.g., by controlling laser apparatus 3, laser beam
direction control unit 34, or the like), for example. The various
controls mentioned above are merely examples, and other controls may be
added as necessary.

3.3 Burst Operation

[0071] The EUV light generation system 11 may supply, to the exposure
apparatus 6, pulsed EUV light 252 at a predetermined repetition rate,
when wafers are exposed in the exposure apparatus 6. While a wafer is
being moved or replaced, or while a mask is being replaced, exposure of a
wafer may be paused or suspended. Accordingly, during the above stated
periods of pause or suspension, the EUV light generation system 11 may
not supply the pulsed EUV light 252 to the exposure apparatus 6.

[0072] In the EUV light generation system 11, the laser apparatus 3 may
need to be controlled in order to cause the EUV light generation system
11 to supply, or to stop supplying, the pulsed EUV light 252.

[0073] The burst operation mode may be one of several different modes or
states of operation of the laser apparatus 3. In the burst operation
mode, a pulsed laser beam may be outputted at a predetermined repetition
rate during a first predetermined period, and the pulsed laser beam may
not be outputted during a second predetermined period. The first and
second periods may be non-overlapping periods, and may be repeated in
alternation. For example, as shown in FIG. 2, a pulsed laser beam of
uniform beam intensity may be outputted at a predetermined repetition
rate during a burst period B, and the pulsed laser beam may not be
outputted during a rest period TR. The pulsed laser beam may also be
outputted during a series of burst periods B (e.g., burst periods that
are repeated periodically), and the pulsed laser beam may not be
outputted during rest periods TR (e.g., rest periods that are repeated
periodically).

[0074] When the laser apparatus 3 is made to oscillate in burst operation
mode, a heat load on the focusing optical system (such as the laser
focusing mirror 22) may undergo larger fluctuations than when the laser
apparatus 3 is made to oscillate continually. As a result of the
fluctuations in heat load, the position at which the pulsed laser beam 33
reflected by the laser focusing mirror 22 is focused may fluctuate.

[0075] Further, the position at which the pulsed laser beam 33 reflected
by the laser focusing mirror 22 is focused may shift due to vibrations of
the laser focusing mirror 22. The fluctuation in the focus position may
be detected by measuring the position at which the pulsed laser beam 33
is focused. However, with this method, it may be difficult (if not
impossible) to detect the fluctuation in the focus position during the
rest period TR, for example during the rest periods TR present when the
laser apparatus 3 is made to operate in the burst operation mode.
Accordingly, when the focus position changes during a rest period TR
during which the focus position was not detected or measured, the lead
pulse of a given burst period B immediately following the rest period TR
may be focused in an unknown or unintended position.

4. EUV Light Generation System of First Embodiment

[0076] A first embodiment of this disclosure will be described in detail
with reference to the drawings.

4.1 Configuration

[0077]FIG. 3 schematically illustrates the configuration of an EUV light
generation system 11A according to the first embodiment. As illustrated
in FIG. 3, the EUV light generation system 11A may include a focusing
optical system for focusing a pulsed laser beam and a beam splitter.
Inside the chamber 2, which constitutes part of the EUV light generation
system 11A, the EUV collector mirror 23, a beam splitter 100, an optical
sensor 110, a first mirror 152, a second mirror 153, and plates 51 and 52
may be disposed. The EUV collector mirror 23 may be attached to the plate
51 via a holder 23a. The first and second mirrors 152 and 153 may be
attached to the plate 52 via holders 152a and 153a, respectively. The
beam splitter 100 may be attached to the plate 52 via a holder 100a so as
to be positioned between the second mirror 153 and the first focus of the
EUV collector mirror 23, and such that the pulsed laser beam 33 reflected
by the second mirror 153 may be incident on the beam splitter 100. The
beam splitter 100 may be configured so as to split a pulsed laser beam 35
from the pulsed laser beam 33 incident on the beam splitter 100. The
optical sensor 110 may be attached to the plate 52 via a holder 110a such
that the pulsed laser beam 35 split by the beam splitter 100 may be
focused on the photosensitive surface of the optical sensor 110. The beam
splitter 100 may preferably be disposed such that the distance between a
given point on the beam splitter 100 and the first focus of the EUV
collector mirror 23 is substantially equal to the distance between the
given point and the photosensitive surface of the optical sensor 110. The
optical sensor 110 may include one or more of an area sensor, a position
sensitive detector (PSD), or the like. The plate 52 may be attached to
the plate 51. The plate 51 may have a through-hole 50 formed therein for
allowing the pulsed laser beam 33 to pass therethrough. High-reflection
mirrors 341 and 342 serving as the laser beam direction control unit 34
may be interposed between the laser apparatus 3 and the chamber 2. A
monitor 120 may be connected to the optical sensor 110, and the monitor
120 may display the output from the optical sensor 110 as image data or
as numerical data.

[0078] The first mirror 152 may include an off-axis paraboloidal convex
mirror, and may serve as a beam expanding optical system for the pulsed
laser beam. The second mirror 153 may be a spheroidal concave mirror, and
may serve as a focusing optical system for the pulsed laser beam. The
first and second mirrors 152 and 153 may jointly be referred to as a beam
expanding and focusing optical system 150 for the pulsed laser beam. With
reference to FIG. 4, the first and second mirrors 152 and 153 may be
disposed such that the focus of a parabola E2 following along the
extension of the reflective surface of the first mirror 152 and one of
the foci of an ellipse E1 following along the extension of the reflective
surface of the second mirror 153 both lie on a focus F1. Further, the
second mirror 153 may be disposed such that the other focus F2 of the
ellipse E1 lies in the plasma generation region 25. Here, FIG. 4
schematically illustrates the configuration of the beam expanding and
focusing optical system which includes a focusing optical system.

4.2 Operation

[0079] The pulsed laser beam 31 outputted from the laser apparatus 3 may
be reflected by the high-reflection mirrors 341 and 342 without being
expanded in diameter, and enter the chamber 2 via the window 21. The
pulsed laser beam 31 may be incident on the first mirror 152 at a
predetermined angle (at an angle of 45 degrees with respect to the
principle axis of the first mirror 152, for example) and be reflected by
the first mirror 152, to thereby be expanded in diameter. The pulsed
laser beam, which has been expanded in diameter, may be incident on the
second mirror 153 at a predetermined angle (at an angle of 45 degrees
with respect to the principle axis of the second mirror 153, for example)
and be reflected by the second mirror 153, to thereby be focused at a
location which coincides with the first focus of the EUV collector mirror
23. Here, the first and second mirrors may be disposed such that the
pulsed laser beam incident on the first mirror 152 is reflected so as to
be incident on the second mirror 153. Part of the pulsed laser beam 33
reflected by the second mirror 153 may be split by the beam splitter 100,
as the pulsed laser beam 35, and the pulsed laser beam 35 may be focused
on the photosensitive surface of the optical sensor 110. The optical
sensor 110 may detect the focus condition of the pulsed laser beam 35 and
output the focus condition information to the monitor 120. Then, the
position and/or orientation of each of the high-reflection mirrors 341
and 342 may be adjusted based on the display on the monitor 120, and the
focus condition of the pulsed laser beam 35 may be adjusted to a desired
condition.

4.3 Effect

[0080] The optical sensor 110 may detect the pulsed laser beam 35, which
is split from the pulsed laser beam 33. By monitoring in real-time the
position at which the pulsed laser beam 35 is focused and the beam
profile (focus condition) of the pulse laser beam 35, the optical sensor
110 may be able to estimate or indirectly monitor, in real-time, the
position at which the pulsed laser beam 33 is focused and the beam
profile (focus condition) of the focused pulsed laser beam 33.

4.4 Modification

[0081] The plate 51 may be configured so as to divide the interior of the
chamber 2 into two spaces, one to the side of the EUV collector mirror 23
and the other to the side of the second mirror 153. In this case, the
through-hole 50 may preferably be covered by a window, through which the
pulsed laser beam 33 may be transmitted. When the interior of the chamber
2 is divided by the plate 51, optical systems on one side of the plate
51, such as the second mirror 153, may be prevented from being
contaminated by debris emitted when the target material is turned into
plasma on the other side of the plate 51.

5. EUV Light Generation System of Second Embodiment

[0082] A second embodiment of this disclosure will be described in detail
with reference to the drawings.

5.1 Configuration

[0083] FIG. 5 schematically illustrates the configuration of an EUV light
generation system 11B according to the second embodiment. As illustrated
in FIG. 5, the EUV light generation system 11B may include a focusing
optical system for the pulsed laser beam, a beam splitter, and a system
for using feedback-control to adjust the focus position of the pulsed
laser beam. Here, the EUV light generation system 11B may be similar in
configuration to the EUV light generation system 11A shown in FIG. 3 and
may further include a focus control unit 160, a focus position adjusting
unit 170, a mount 180 equipped with a tilt mechanism, and a plate 53. The
focus position adjusting unit 170 may be disposed on the beam path
between the high-reflection mirrors 341 and 342. The high-reflection
mirror 342 may be held by the mount 180. The focus position adjusting
unit 170 and the mount 180 may be attached to the plate 53. The focus
control unit 160 may be connected to and receive input from the optical
sensor 110. The focus control unit 160 may be connected to and transmit
control commands to the focus position adjusting unit 170 and the mount
180.

5.2 Operation

[0084] The pulsed laser beam 35 may be focused on the photosensitive
surface of the optical sensor 110. The focus condition information of the
pulsed laser beam 35 detected by the optical sensor 110 may be inputted
to the focus control unit 160. Based on the inputted focus condition
information, the focus control unit 160 may calculate or estimate the
focus position and the beam profile of the pulsed laser beam 35. Then,
based on the calculated or estimated focus position and beam profile, the
focus control unit 160 may use feedback-control to adjust the focus
position using the adjusting unit 170 and the mount 180 so that the
pulsed laser beam 35 can be focused in a desired condition. The focus
position adjusting unit 170 may adjust the wavefront of the pulsed laser
beam 31 incident thereon, to thereby adjust the focus position of the
pulsed laser beam 33 in the direction of the beam axis and the beam
profile. The focus position adjusting unit 170 may further be used to
adjust at least one of a beam axis (including a position of a beam axis
and/or a direction of the beam axis) and divergence of the laser beam.
The mount 180 may function as a beam axis adjusting unit. In that case,
the mount 180 may adjust the direction of the beam axis of the pulsed
laser beam 31, to thereby adjust the focus position of the pulsed laser
beam 33. The direction of the beam axis of the pulsed laser beam 31 may
be adjusted by causing the mount 180 to change an orientation of the
high-reflection mirror 342, for example.

5.3 Effect

[0085] The focus condition of the pulsed laser beam 35 detected by the
optical sensor 110 may reflect the focus condition of the pulsed laser
beam 33. Accordingly, the focus control unit 160 may, by using
feedback-control to adjust the focus condition of the pulsed laser beam
35, be able to adjust the focus condition of the pulsed laser beam 33 in
real-time. For example, the focus control unit 160 may use
feedback-control to adjust the focus position of the pulsed laser beam 33
to a desired focus position. Further, the focus control unit 160 may
control the focus position of the pulsed laser beam 33 so as to move the
focus position to a desired focus position.

6. EUV Light Generation System of Third Embodiment

[0086] A third embodiment of this disclosure will be described in detail
with reference to the drawings.

6.1 Configuration

[0087] FIG. 6 schematically illustrates the configuration of an EUV light
generation system 11C according to the third embodiment. As illustrated
in FIG. 6, the EUV light generation system 11C may include a guide laser,
a focusing optical system for a pulsed laser beam, and a beam splitter.
Here, the EUV light generation system 11C may include a beam path
adjusting unit 220 in place of the high-reflection mirror 341 in the EUV
light generation system 11A shown in FIG. 3. Further, the EUV light
generation system 11C may include a guide laser 200 and a beam expander
210. The guide laser 200 may be configured to output a continuous-wave
guide laser beam even during the rest period TR of the burst operation.
The beam expander 210 may be configured to expand the guide laser beam
incident thereon in diameter, collimate the guide laser beam, and output
the collimated guide laser beam as a guide laser beam 41. The optical
sensor 110 may be configured to detect the guide laser beam 41 incident
thereon. The beam path adjusting unit 220 may include a beam combiner
that reflects the pulsed laser beam 31 with high reflectivity on a
surface thereof and transmits the guide laser beam 41 through the
adjusting unit. The beam path adjusting unit 220 may be coated with an
optical thin film, which reflects the pulsed laser beam 31 with high
reflectivity and transmits the guide laser beam 41 with high
transmissivity, on a surface on which the pulsed laser beam 31 is
incident. Further, the beam path adjusting unit 220 may be coated with an
optical thin film, which transmits the guide laser beam 41 with high
transmissivity, on a surface on which the guide laser beam 41 is
incident. The high-reflection mirror 342, and the first and second
mirrors 152 and 153 may respectively be coated with optical thin films on
the respective reflective surfaces thereof, for reflecting the pulsed
laser beam and the guide laser beam with high reflectivity. The beam
splitter 100 may be formed of a diamond substrate. The beam splitter 100
may be coated with an optical thin film, on a surface facing the second
mirror 153, for reflecting the guide laser beam with high reflectivity
and transmitting the pulsed laser beam with high transmissivity. Further,
the beam splitter 100 may be coated with an optical thin film, on a
surface oriented toward the EUV collector mirror 23, for transmitting the
pulsed laser beam and the guide laser beam with high transmissivity. The
window 21 may be formed of a diamond substrate and may be coated with an
optical thin film, on both principal surfaces thereof, for transmitting
the pulsed laser beam and the guide laser beam with high transmissivity.

6.2 Operation

[0088] The beam path of the pulsed laser beam 31 and the beam path of the
guide laser beam 41 may be made to substantially coincide with each other
by the beam path adjusting unit 220. A laser beam 42 outputted from the
beam path adjusting unit 220 may include the pulsed laser beam 31 and the
guide laser beam 41. The laser beam 42 may be reflected by the
high-reflection mirror 342, transmitted through the window 21, and may
enter the chamber 2. The laser beam 42 may be incident on the first
mirror 152 at 45 degrees and be reflected thereby. By being reflected on
the paraboloidal convex surface of the first mirror 152, the laser beam
42 may be expanded in diameter. The laser beam 42 reflected by the first
mirror 152 may be incident on the second mirror 153 at 45 degrees. The
laser beam 42 reflected by the second mirror 153 may be focused in the
first focus of the EUV collector mirror 23. The beam splitter 100 may
reflect the guide laser beam 41 of the laser, beam 42 with high
reflectivity, and transmit the pulsed laser beam 31 of the laser beam 42.
The laser beam 42 may thus be split into a guide laser beam 44,
corresponding to the reflected part of laser beam 42, and the pulsed
laser beam 33, corresponding to the transmitted part of the laser beam
42. The guide laser beam 44 may be focused on the photosensitive surface
of the optical sensor 110. The optical sensor 110 may detect and/or
measure properties of the focused guide laser beam 44 and output the
detection and measuring result to the monitor 120. In one example, the
wavelength of the pulsed laser beam 31 differs from the wavelength of the
guide laser beam 41. The second mirror 153 serving as the focusing
optical system may be of a reflective type. In that case, chromatic
aberration may not occur even when the wavelengths of the reflected laser
beams differ from each other.

6.3 Effect

[0089] The optical sensor 110 may detect and/or measure properties of the
guide laser beam 44 split by the beam splitter 100. The focus condition
information of the detected guide laser beam 44 may include information
on the focus condition of the pulsed laser beam 33. Accordingly, as the
optical sensor 110 detects and/or measure properties of the guide laser
beam 44, the focus position and the beam profile of the pulsed laser beam
33 may be detected or estimated. As a result, the optical sensor 110 may
be able to monitor, in real-time, the position at which the pulsed laser
beam 33 is focused and the beam profile (focus condition) of the focused
pulsed laser beam 33.

[0090] Further, the guide laser beam 44 may be outputted even during the
rest period TR of the burst operation. Accordingly, using the guide laser
beam 44 to monitor and control the focus condition of the pulsed laser
beam 33 may allow the focus position of the pulsed laser beam 33 to be
detected even during the rest period TR. As a result, the focus condition
of the lead pulse in a burst period B immediately following a rest period
TR may be controlled to a desired condition.

7. EUV Light Generation System of Fourth Embodiment

[0091] A fourth embodiment of this disclosure will be described in detail
with reference to the drawings.

7.1 Configuration

[0092]FIG. 7 schematically illustrates the configuration of an EUV light
generation system 11D according to the fourth embodiment. As illustrated
in FIG. 7, the EUV light generation system 11D may include a guide laser,
a focusing optical system for the pulsed laser beam, a beam splitter, and
a system for using feedback-control to adjust the focus position of the
pulsed laser beam. Here, the EUV light generation system 11D may be
similar in configuration to the EUV light generation system 11C shown in
FIG. 6 and may further include the focus control unit 160, the focus
position adjusting unit 170, and the mount 180 equipped with a tilt
mechanism included in the EUV light generation system 11B shown in FIG.
5, and a plate 53. The focus position adjusting unit 170 may be disposed
on the beam path between the beam path adjusting unit 220 and the
high-reflection mirror 342. The high-reflection mirror 342 may be held by
the mount 180. The focus position adjusting unit 170 and the mount 180
may be attached to the plate 53. The focus control unit 160 may be
connected to the optical sensor 110. The focus control unit 160 may also
be connected to the focus position adjusting unit 170 and the mount 180.

7.2 Operation

[0093] The guide laser beam 44 split by the beam splitter 100 may be
focused on the photosensitive surface of the optical sensor 110. The
focus condition information of the guide laser beam 44 detected and/or
measured by the optical sensor 110 may be inputted to the focus control
unit 160. Based on the inputted focus condition information, the focus
control unit 160 may obtain or estimate the focus position and the beam
profile of the pulsed laser beam 33 from the focus position and the beam
profile of the guide laser beam 44. Then, based on the obtained or
estimated focus position and beam profile, the focus control unit 160 may
use feedback-control to control the focus position adjusting unit 170 and
adjust the mount 180. With this, the focus condition of the pulsed laser
beam 33 may be adjusted to a desired condition. The focus position
adjusting unit 170 may adjust the wavefront of the laser beam incident
thereon, to thereby adjust the focus position of the incident laser beam
in the direction of the beam axis and the beam profile. The focus
position adjusting unit 170 may further be used to adjust at least one of
a beam axis (including a position of a beam axis and/or a direction of
the beam axis) and divergence of the laser beam. The mount 180 may
function as a beam axis adjusting unit. In that case, the mount 180 may
adjust the orientation of the high-reflection mirror 342 to as to adjust
the direction of the beam axis of the pulsed laser beam 31, to thereby
adjust the focus position of the pulsed laser beam 33.

7.3 Effect

[0094] The focus condition of the guide laser beam 44 detected or measured
by the optical sensor 110 may substantially reflect the focus condition
of the pulsed laser beam 33. Accordingly, the focus control unit 160 may
be able to use feedback-control to adjust the focus condition of the
pulsed laser beam 33 in real-time. That is, the focus control unit 160
may use feedback-control based on the focus condition of the guide laser
beam 44 detected or measured by the optical sensor 110 to adjust the
focus condition of the pulsed laser beam 33. For example, the focus
control unit 160 may use feedback-control to adjust the focus position of
the pulsed laser beam 33 to a desired position. Further, the focus
control unit 160 may control the focus position of the pulsed laser beam
33 so as to move the focus position to a desired position.

[0095] Further, the guide laser beam 44 may be outputted even during the
rest period TR of the burst operation. Accordingly, using the guide laser
beam 44 may allow the focus position of the pulsed laser beam 33 to be
detected even during the rest period TR. As a result, the focus condition
of the lead pulse in a burst period B immediately following a rest period
TR may be controlled to a desired condition.

8. Beam Splitters

[0096] Variations of the beam splitter 100 will be described in detail
with reference to the drawings.

8.1 Diamond Beam Splitter

[0097] FIG. 8 schematically illustrates the configuration of a laser beam
focusing optical system in which a diamond beam splitter is used. In the
configuration shown in FIG. 8, a third mirror 154 including an off-axis
paraboloidal mirror is provided in place of the first and second mirrors
152 and 153, and the laser beam 42 reflected by the high-reflection
mirror 342 may be directly incident on the third mirror 154. A beam
splitter 101, corresponding to the beam splitter 100, may be a diamond
beam splitter formed of a diamond substrate. The laser beam 42 may
include the guide laser beam, and the beam splitter 101 may be coated
with an optical thin film on a surface facing the EUV collector mirror
23. The optical thin film may be configured to reflect the pulsed laser
beam 33 component of the laser beam 42 with high reflectivity and
transmit the guide laser beam component of the laser beam 42 with high
transmissivity. The beam splitter 101 may be coated with an optical thin
film, on a surface facing the optical sensor 110, for transmitting the
guide laser beam component of the laser beam 42 with high transmissivity.
Using the beam splitter 101 formed of a diamond substrate may make it
possible to suppress thermal deformation in the beam splitter 101.

8.2 Mirror Having Through-Hole

[0098] As illustrated in FIGS. 9 and 10, in place of the beam splitter
100, a beam splitter 102 having a through-hole 102a formed at the center
thereof may be used. In this case, the guide laser beam 41 may be
expanded in diameter by the beam expander 210 such that the cross-section
of the guide laser beam 41 is larger than the cross-section of the pulsed
laser beam 31. FIG. 9 schematically illustrates the configuration of a
laser beam focusing optical system, in which a mirror having a
through-hole according to another modification of the beam splitter is
used, and FIG. 10 illustrates the shape of the mirror serving as the beam
splitter shown in FIG. 9.

[0099] Here, the beam splitter 102 may be disposed such that the pulsed
laser beam 33 included in the laser beam 42 passes through the
through-hole 102a formed in a region Ell of the beam splitter 102. Parts
of the guide laser beam that are outside of the pulsed laser beam may be
reflected by a peripheral region E12 of the beam splitter 102. A guide
laser beam 45 split from the laser beam 42 by the beam splitter 102 may
be annular in cross-section and be focused on the photosensitive surface
of the optical sensor 110. Meanwhile, the guide laser beam that has
passed through the through-hole 102a may be focused in the plasma
generation region 25.

[0100] The beam splitter 102 may have the through-hole 102a formed
therein. In general, the beam splitter 102 may be circular (or annular,
and having circular inner and outer edges), and the through-hole 102
formed therein may also be circular. In some embodiments, however, the
beam splitter 102 and through-hole may have oval shapes. Since the pulsed
laser beam passes through the through-hole 102a, the pulsed laser beam is
not reflected by the beam splitter 102. Accordingly, the beam splitter
102 may not be heated by the pulsed laser beam; thus, thermal deformation
in the beam splitter 102 may be suppressed and the guide laser beam may
be detected with high precision. Additionally, the pulsed laser beam is
not reflected by the beam splitter 102, and is incident on the plasma
generation region 25.

8.3 Diffraction Grating

[0101]FIG. 11 schematically illustrates the configuration of a laser beam
focusing optical system, in which a diffraction grating according to yet
another modification of the beam splitter is used. As illustrated in FIG.
11, in place of the beam splitter 100, a beam splitter 103 having a
diffraction grating may be used. Here, the beam splitter 103 may be
mounted to the reflective surface of the second mirror 153. That is, a
diffraction grating spheroidal concave focusing mirror 155 may include
the beam splitter 103 and the second mirror 153.

[0102] A gap between two adjacent slits on the diffraction grating of the
beam splitter 103 may be smaller than the wavelength of the pulsed laser
beam and larger than the wavelength of the guide laser beam.

[0103] When the beam splitter 103 is used, the pulsed laser beam may be
reflected at the reflective surface of the second mirror 153 with high
reflectivity and be focused in the plasma generation region 25 without
being diffracted by the beam splitter 103. Meanwhile, the guide laser
beam may be diffracted by the beam splitter 103 and be focused on the
photosensitive surface of the optical sensor 110. Accordingly, the
optical sensor 110 may be disposed at a position at which the diffracted
guide laser beam is focused. The diffraction grating spheroidal concave
focusing mirror 155 may be equipped with both a focusing function and a
beam splitting function; thus, the number of optical elements can be
reduced and the laser beam focusing optical system may be made smaller in
size.

9. EUV Light Generation System of Fifth Embodiment

[0104] A fifth embodiment of this disclosure will be described in detail
with reference to the drawings.

9.1 Configuration

[0105] FIG. 12 schematically illustrates the configuration of an EUV light
generation system 11E according to the fifth embodiment. As illustrated
in FIG. 12, the EUV light generation system 11E may include a guide
laser, a focusing optical system for the pulsed laser beam, a beam
splitter, and a system for using feedback-control to adjust the focus
position of the pulsed laser beam. The EUV light generation system 11E
shown in FIG. 12 may include a guide laser beam mirror 201 in addition to
the configuration of the EUV light generation system 11D shown in FIG. 7.
Further, in the configuration shown in FIG. 12, the beam splitter 100
shown in FIG. 7 may be replaced by a high-reflection mirror 100E.
Furthermore, in the configuration shown in FIG. 12, the optical sensor
110 may be disposed so as to be located opposite from and facing the
guide laser beam mirror 201, with the high-reflection mirror 100E located
between the optical sensor 110 and guide laser beam mirror 201.

[0106] The high-reflection mirror 100E may reflect the pulsed laser beam
33 with high reflectivity. The high-reflection mirror 100E may reflect
part of the guide laser beam 44 and transmit another part thereof. The
guide laser beam 44 transmitted through the high-reflection mirror 100E
may be a so-called leak beam of the guide laser beam 44.

[0107] As illustrated in FIG. 13, the guide laser beam mirror 201 may have
a through-hole 201b formed at the center thereof. In one example, the
guide laser beam mirror 201 is circular (or annular, with circular inner
and outer edges) and the through-hole 201b is circular. However, without
being limited thereto, a mirror (for example, a dichroic mirror) that
transmits the pulsed laser beam 33 and reflects the guide laser beam 44
may be used. The guide laser beam mirror 201 may be attached to the back
surface of the EUV collector mirror 23, so as to surround the
through-hole 24, via a holder 201a. Here, the through-hole 201b may
overlap with the through-hole 24, when viewed from the side of the
high-reflection mirror 100E.

[0108] The optical sensor 110 may preferably be disposed so as to be
distanced by a predetermined distance from the guide laser beam mirror
201. Here, the predetermined distance is preferably equal to the distance
between the first focus of the EUV collector mirror 23 and the reflective
surface of the guide laser beam mirror 201. Further, the optical sensor
110 may preferably be disposed on an extension of an axis connecting the
first focus of the EUV collector mirror 23 and the center of the guide
laser beam mirror 201.

[0109] In the configuration shown in FIG. 12, the optical system for
expanding the laser beam 42 in diameter (the first mirror 152 and the
holder 152a shown in FIG. 7) may be omitted, and the laser apparatus 3
may be configured to output the pulsed laser beam 31 which is expanded in
diameter. However, in other embodiments in which the laser apparatus 3
may not output a pulsed laser beam 31 having an expanded diameter, the
optical system for expanding the laser beam 42 in diameter may be
included as part of the optical system 11E and disposed upstream from the
second mirror 153. As shown, the beam expander 210 disposed downstream
from the guide laser 200 may preferably expand the guide laser beam 41 in
diameter such that the cross-section of the guide laser beam 41 is larger
than the cross-section of the pulsed laser beam 31.

9.2 Operation

[0110] In some embodiments, it may be preferable that the cross-section of
the guide laser beam 41, which has been expanded in diameter by the beam
expander 210, be larger than the cross-section of the pulsed laser beam
31. The beam axis of the guide laser beam 41 may be made to coincide with
the beam axis of the pulsed laser beam 32 as the guide laser beam passes
through the beam path adjusting unit 220.

[0111] The pulsed laser beam 33 reflected by the second mirror 153 may be
reflected by the high-reflection mirror 100E. The pulsed laser beam 33
reflected by the high-reflection mirror 100E may travel through the
through-hole 201b formed in the guide laser beam mirror 201 and the
through-hole 24 formed in the EUV collector mirror 23 and be focused in
the plasma generation region 25. Meanwhile, the guide laser beam 44
reflected by the second mirror 153 may be reflected by the
high-reflection mirror 100E. At least an edge portion of the guide laser
beam 44 reflected by the high-reflection mirror 100E may be reflected by
the guide laser beam mirror 201, which is annular in shape. The edge
portion of the guide laser beam 44 reflected by the guide laser beam
mirror 201 may be incident on the high-reflection mirror 100E. The
high-reflection mirror 100E may reflect part of the guide laser beam 44,
but may transmit part of the guide laser beam 44 toward the optical
sensor 110 disposed behind the high-reflection mirror 100E. Here, a
returning beam of the guide laser beam 44 reflected by the guide laser
beam mirror 201 may possibly be generated, but the intensity of this
returning beam is weak and may generally not cause a problem.

[0112] The guide laser beam 44 reflected by the guide laser beam mirror
201 and transmitted through the high-reflection mirror 100E may be
focused on the photosensitive surface of the optical sensor 110. The
focus condition information of the guide laser beam 44 detected by the
optical sensor 110 may be inputted to the focus control unit 160. Based
on the inputted focus condition information, the focus control unit 160
may obtain the focus position and the beam profile of the guide laser
beam 44, and may use the obtained focus position and beam profile of the
guide laser beam 44 to estimate the focus position and the beam profile
of the pulsed laser beam 33. Then, based on the obtained or estimated
focus position and beam profile, the focus control unit 160 may use
feedback-control to adjust the focus position adjusting unit 170 and
control the orientation of the mount 180. With this, the focus condition
of the pulsed laser beam 33 may be adjusted to a desired condition. The
focus position adjusting unit 170 may adjust the wavefront of the laser
beam incident thereon, to thereby adjust the focus position of the pulsed
laser beam 33 in the direction of the beam axis and the beam profile. The
focus position adjusting unit 170 may further be used to adjust at least
one of a beam axis (including a position of a beam axis and/or a
direction of the beam axis) and divergence of the laser beam. The mount
180 may function as a beam axis adjusting unit. In that case, the
orientation of the mount 180 may be adjusted to control the direction of
the beam axis of the pulsed laser beam 33, to thereby adjust the focus
position.

9.3 Effect

[0113] The focus condition of the guide laser beam 44 detected by the
optical sensor 110 may substantially reflect (or be indicative of) the
focus condition of the pulsed laser beam 33. Accordingly, the focus
control unit 160 may be able to use feedback-control to adjust the focus
condition of the pulsed laser beam 33 in real-time. That is, the focus
control unit 160 may use feedback-control based on the obtained focus
condition of the guide laser beam 44 to adjust the focus condition of the
pulsed laser beam 33. For example, the focus control unit 160 may use
feedback-control to adjust the focus position of the pulsed laser beam 33
to a desired position. Further, the focus control unit 160 may control
the focus position of the pulsed laser beam 33 so as to move to a desired
position.

[0114] Further, the guide laser beam 44 may be outputted even during the
rest period TR of the burst operation. Accordingly, using the guide laser
beam 44 may allow the focus position of the pulsed laser beam 33 to be
detected or estimated even during the rest period TR. As a result, the
focus condition of the lead pulse in a burst period B immediately
following a rest period TR may be controlled to a desired condition.

[0115] Further, the guide laser beam mirror 201 may be attached to the EUV
collector mirror 23 via the holder 201a. In that case, a change in the
direction and the relative position of the pulsed laser beam 33 (and the
guide laser beam 44) with respect to the EUV collector mirror 23 may
cause a change in the focus position of the edge portion of the guide
laser beam 44 reflected by the guide laser beam mirror 201. The change in
focus position of the edge portion of the guide laser beam 44 may be
detected by the optical sensor 110. Accordingly, based on this detection
result, the direction of the laser beam 42 may be controlled. With this,
the focus position of the pulsed laser beam 33 may be controlled to the
first focus of the EUV collector mirror 23 (and/or to the plasma
generation region 25) with high precision. In this way, even when the
first focus of the EUV collector mirror 23 is positioned outside of the
predetermined plasma generation region 25 due to the EUV collector mirror
23 being moved, for example, the plasma generation region 25 may be
re-set to the moved first focus of the EUV collector mirror 25.

10. EUV Light Generation System of Sixth Embodiment

[0116] A sixth embodiment of this disclosure will be described in detail
with reference to the drawings. In FIG. 14, however, the elements similar
to those shown in FIG. 7 are omitted.

10.1 Configuration

[0117]FIG. 14 schematically illustrates the configuration of an EUV light
generation system 11F according to the sixth embodiment. As illustrated
in FIG. 14, the EUV light generation system 11F may include a guide
laser, a focusing optical system for the pulsed laser beam, a beam
splitter, and a system for using feedback-control to adjust the focus
position of the pulsed laser beam. Here, the EUV light generation system
11F shown in FIG. 14 may include a guide laser beam mirror 202 in
addition to the configuration of the EUV light generation system 11D
shown in FIG. 7. Further, in the configuration shown in FIG. 14, the beam
splitter 100 and the holder 100a shown in FIG. 7 may be omitted.

[0118] The guide laser beam mirror 202 may be attached to the back surface
of the EUV collector mirror 23, so as to surround at least part of the
through-hole 24, via a holder 202a. The guide laser beam mirror 202 may
be sectoral in shape following along the edge (or a portion of the edge)
of the through-hole 24. In one example, the guide laser beam mirror 202
has the shape of a sector of an annulus, such as a sector of a guide
laser beam mirror 201 shown in FIG. 13. The guide laser beam mirror 202
can be an annulus sector having an angle of, for example, 45°,
60°, 90°, 135°, 180°, 270°, or any
other appropriate sector angle. However, the shape of the guide laser
beam mirror 202 is not limited thereto.

[0119] The beam expander 210 disposed downstream from the guide laser 200
may preferably expand the guide laser beam 41 in diameter such that the
cross-section of the guide laser beam 41 is larger than the cross-section
of the pulsed laser beam 31. With this, part of the edge portion of the
guide laser beam 44 reflected by the second mirror 153 may be incident on
the guide laser beam mirror 202.

[0120] The optical sensor 110 may preferably be disposed such that the
part of the guide laser beam 44 reflected by the guide laser beam mirror
202 is focused on the photosensitive surface of the optical sensor 110.

10.2 Operation

[0121] In some embodiments, it may be preferable that the cross-section of
the guide laser beam 41, which has been expanded in diameter by the beam
expander 210, be larger than the cross-section of the pulsed laser beam
31. The beam axis of the guide laser beam 41 may be made to coincide with
the beam axis of the pulsed laser beam 32 as the guide laser beam 41
passes through the beam path adjusting unit 220.

[0122] The pulsed laser beam 33 reflected by the second mirror 153 may
travel through the through-hole 24 formed in the EUV collector mirror 23
and be focused in the plasma generation region 25. Meanwhile, at least
part of an edge portion of the guide laser beam 44 reflected by the
second mirror 153 may be reflected by the guide laser beam mirror 202
disposed to surround at least part of the through-hole 24.

[0123] The part of the edge portion of the guide laser beam 44 reflected
by the guide laser beam mirror 202 may be focused on the photosensitive
surface of the optical sensor 110. The focus condition information of the
guide laser beam 44 detected by the optical sensor 110 may be inputted to
the focus control unit 160 (See FIG. 12). Based on the inputted focus
condition information, the focus control unit 160 may obtain the focus
position and the beam profile of the guide laser beam 44 and estimate the
focus position and the beam profile of the pulsed laser beam 33. Then,
based on the obtained or estimated focus position and beam profile, the
focus control unit 160 may use feedback-control to control the focus
position adjusting unit 170 and the mount 180 (See FIG. 12). With this,
the focus condition of the pulsed laser beam 33 may be adjusted to a
desired condition. The focus position adjusting unit 170 may adjust the
wavefront of the laser beam incident thereon, to thereby adjust the focus
position of the pulsed laser beam 33 in the direction of the beam axis
and the beam profile. The focus position adjusting unit 170 may further
be used to adjust at least one of a beam axis (including a position of a
beam axis and/or a direction of the beam axis) and divergence of the
laser beam. The mount 180 may function as a beam axis adjusting unit. In
that case, the mount 180 may adjust the direction of the beam axis of the
pulsed laser beam 33, to thereby adjust the focus position.

10.3 Effect

[0124] The focus condition of the guide laser beam 44 detected by the
optical sensor 110 may substantially be indicative of the focus condition
of the pulsed laser beam 33. Accordingly, the focus control unit 160 may
use feedback-control to adjust the focus condition of the pulsed laser
beam 33 in real-time. That is, the focus control unit 160 may use
feedback-control to adjust the focus condition of the pulsed laser beam
33. For example, the focus control unit 160 may use feedback-control to
adjust the focus position of the pulsed laser beam 33 to a desired
position. Further, the focus control unit 160 may control the focus
position of the pulsed laser beam 33 so as to move to a desired position.

[0125] Further, the guide laser beam 44 may be outputted even during the
rest period TR of the burst operation. Accordingly, using the guide laser
beam 44 may allow the focus position of the pulsed laser beam 33 to be
detected or estimated even during the rest period TR. As a result, the
focus condition of the lead pulse in a burst period B immediately
following a rest period TR may be controlled to a desired condition.

[0126] Further, the guide laser beam mirror 202 may be attached to the EUV
collector mirror 23 via the holder 202a. In that case, a change in the
direction and the relative position of the pulsed laser beam 33 (and the
guide laser beam 44) with respect to the EUV collector mirror 23 may
cause a change in the focus position of the part of the edge portion of
the guide laser beam 44 reflected by the guide laser beam mirror 202. The
change in focus position of the part of the edge portion of the guide
laser beam 44 may be detected by the optical sensor 110. Accordingly,
based on this detection result, the direction of the laser beam 42 may be
controlled. With this, the focus position of the pulsed laser beam 33 may
be controlled to the first focus of the EUV collector mirror 23 (and/or
to the plasma generation region 25) with high precision. In this way,
even when the first focus of the EUV collector mirror 23 is positioned
outside of the plasma generation region 25 due to the EUV collector
mirror 23 being moved, for example, the plasma generation region 25 may
be re-set to the moved first focus of the EUV collector mirror 25.

11. Supplementary Description

11.1 Mount Having Tilt Mechanism

[0127]FIG. 15 is a perspective view illustrating an example of the mount
180 shown in FIGS. 5 and 7. As illustrated in FIG. 15, the mount 180 may
include a holder 181, to which the flat high-reflection mirror 342 is
attached, and three automatic micrometers 182 through 184, for example.
Retaining the holder 181 adjustably with the automatic micrometers 182
through 184 may make it possible to adjust the angle θx in
X-direction and the angle θy in Y-direction of the high-reflection
mirror 342 attached to the holder 181.

11.2 Focus Position Adjusting Unit

[0128]FIG. 16 illustrates an example of the focus position adjusting unit
170 shown in FIGS. 5 and 7. As illustrated in FIG. 16, the focus position
adjusting unit 170 may include high-reflection mirrors 61 and 62 and
off-axis paraboloidal concave mirrors 63 and 65. The high-reflection
mirror 62 and the off-axis paraboloidal concave mirror 63 may be attached
to a stage 64, which is movable with respect to the high-reflection
mirror 61 and the off-axis paraboloidal concave mirror 65, for example.
Moving the stage 64 to adjust the distance between the off-axis
paraboloidal concave mirrors 63 and 65 may make it possible to adjust the
wavefront of the pulsed laser beam 31 incident on the focus position
adjusting unit 170 to a predetermined wavefront.

11.3 Modification of Focus Position Adjusting Unit

[0129] Further, the focus position adjusting unit 170 may be modified as
shown in FIGS. 17 through 19. FIGS. 17 through 19 illustrate an example
of a focus position adjusting unit 170A according to a modification. As
illustrated in FIGS. 17 through 19, the focus position adjusting unit
170A may include a deformable mirror 66 having a reflective surface with
a curvature that may be modified. The deformable mirror 66 may reflect
the collimated pulsed laser beam 31 incident thereon as a collimated
laser beam, when the reflective surface thereof is adjusted to be flat,
as illustrated in FIG. 17. The deformable mirror 66, when the curvature
of the reflective surface thereof is adjusted to be concave, may reflect
the collimated pulsed laser beam 31 incident thereon such that the pulsed
laser beam 31 is focused at a predetermined focus F12 distanced therefrom
by a focal distance +F, as illustrated in FIG. 18. Alternatively, the
deformable mirror 66, when the curvature of the reflective surface
thereof is adjusted to be convex, may reflect the collimated pulsed laser
beam 31 incident thereon as a convex beam such that the pulsed laser beam
31 may be focused at a virtual focus F13 distanced therefrom by a focal
distance -F, as illustrated in FIG. 19. As has been described so far,
using the deformable mirror 66 having a reflective surface with a
curvature that may be modified, may make it possible to adjust the
wavefront of the reflected laser beam to a predetermined wavefront in
accordance with the wavefront of the laser beam incident thereon.

[0130] The above-described embodiments and the modifications thereof are
merely examples for implementing this disclosure, and this disclosure is
not limited thereto. Making various modifications according to the
specifications or the like is within the scope of this disclosure, and
other various embodiments are possible within the scope of this
disclosure. For example, the modifications illustrated for particular
ones of the embodiments can be applied to other embodiments as well
(including the other embodiments described herein).

[0131] The terms used in this specification and the appended claims should
be interpreted as "non-limiting." For example, the terms "include" and
"be included" should be interpreted as "including the stated elements but
not limited to the stated elements." The term "have" should be
interpreted as "having the stated elements but not limited to the stated
elements." Further, the modifier "one (a/an)" should be interpreted as
"at least one" or "one or more."